This invention relates to a unique insulating enclosure for low voltage current limiting circuit breakers and, more specifically, to the interconnection between the case and the cover of the enclosure to allow the enclosure to strongly resist the overpressure caused by gases generated during the interruption process.
As already known, particularly high circuit currents generate gas pressure in the order of several P.S.I. in the arc chambers of current limiting circuit breakers due to gases generated by the contacts of the circuit breakers as well as the other components contributing to the arc extinction process.
The standard insulating molded plastic enclosure for such low voltage circuit breakers consists of an enclosure, which contains the circuit breaker components, provided with a cover having openings for an operating handle.
The cover is usually attached to the base by screws passing through the cover itself to engage threaded openings in the case. Additionally, the cover includes projecting ridges for accurate alignment with the case.
The high gas pressure, generated when a short circuit occurs, exerts a strong mechanical stress to the side walls of the case which ordinarily is incapable of resisting such high pressure. Accordingly, it is considered beneficial to transfer such stress from the side walls of the case to the cover which is more capable of resisting stress because of the reduced height and increased thickness of the cover.
One prior attempt to transfer the stress from the circuit breaker case to the cover provided a step on the upper edge of the side walls of the case having a lower portion towards the exterior of the case and an upper portion towards the interior of the case. A complementary step was also provided on the lower edge of the side walls of the cover. This arrangement allowed for cases and covers of equal thickness but required a decrease in the thickness of the steps themselves which ultimately reduced the overall resistance of the enclosure to stress. Increasing the thickness of the steps caused the overall dimensions of the enclosure to become increased and also interfered with the rapid venting of the gases to the exterior of the enclosure.
Another known method of reinforcing the enclosure consisted of providing a number of metal pins in the edge of the cover received within corresponding recesses formed in the cross-section of the side walls of the case. The method was effective for increasing the resistance of the enclosure to stress but resulted in an increase in the overall cost of manufacturing the cover and the case.